1. Field of the Invention
This invention relates to a communications device, method thereof, a communications system and a recording medium, and relates in particular to a communications device, method thereof, a communications system and a recording medium to transmit and receive information for control exceeding one byte.
2. Related Art
Decoders for applications utilizing digital audio and images, perform decoding for instance of data output from an information generator source such as encoders, and of encoded and recorded data, while synchronizing with a clock pulse. If the decoder were capable of controlling the information generator source, then the decoder could adjust the data rate of the data that was sent and thus be dependent only on the timing of the received data, and then decode the data, and display the image or reproduce the audio.
However, when data is sent to the decoder by way for instance of a network, and the decoder possesses no control of the information generator source, the information generator source and the decoder are operated by respective, independent system clocks. The encoding, transmission, receiving, decoding and display processing are therefore performed by the information source generator and decoder based on their respective system clocks. The system clocks in such cases, do not have a common clock to refer to, so that a deviation or discrepancy occurs between the respective system clock frequencies of the information generator source and the decoder.
In this way, when two system clocks are not synchronized, the data sent from the information generator source, and the data decoded by the decoder have different data rates, and the decoder receive buffer overflows or underflows and so the data that was sent is lost. Therefore, when image data for instance is being received, and an underflow occurs, the receiver must again display the frame.
One method to synchronize the decoder system clock with the information generator source system clock is by utilizing information showing the time (time stamp). In an MPEG transport stream, a PCR (Program Clock Reference) is used as a time stamp so that the respective system clocks can be synchronized with each other.
The concept of synchronization in the MPEG transport stream is shown in FIG. 1. The information generator source (transmit side) is operated by a corresponding system clock 1 at a specified frequency. Fixed intervals are not required but the value of a counter counting each system clock 1 is latched at each period, and that counter value is sent to the decoder (receive side). This value is called the time stamp, and the decoder uses this value to synchronize its own system clock 2 with the system clock 1 of the information generator source. More specifically, the count value and the time stamp that was received, are compared in the decoder and based on the comparison results, the system clock 2 on the receive side is made to speed up or slow down.
Synchronization methods such as this are utilized in the MPEG-2 system layer (ISO/IEC13818-1) and ITU-T report. In MPEG-2, a 27 MHz system clock is used in the encoder and decoder. The structure of a system for transmitting data configured for MPEG-2 over a network is shown in FIG. 2. The information generator source is for instance an encoder device such as the encoder 11. The pre-encoded data is accumulated in a data storage device, and this data storage device can be used as the information generator source.
The MPEG-2 data generated in the encoder 11 is input to the system encoder 12. The system encoder 12 adds a time stamp to the MPEG-2 data generated in the encoder 11, packetizes the data, and performs multiplexing to generate an MPEG transport stream packet (hereafter called transport stream packet). An MPEG/ATM converter 13 converts the transport stream packet to ATM cells, and sends them to the receive side by way of the network 14. The transport stream packet is susceptible to various effects of delay distortion inside the network 14 when sending to the receive side. The generation of the delay distortion in the ATM is related later on.
The transport stream (time stamp) containing delay distortion is packetized in the MPEG converter 15, input to the system decoder 16 and processed in the system decoder 16. The decoder system clock is reproduced from the time stamp in this way. A 27 MHz system clock is reproduced in the case of MPEG-2. The transport stream packet processing by the system decoder 16 is output to the decoder 17 where decoding into MPEG-2 data is performed.
Two streams referred to as the program stream and the transport stream are present in the MPEG-2 system layer. The program stream is available for systems without errors such as storage media. The transport stream is available for systems having errors such as communications. The time stamp in the program stream is referred to as the SCR (system clock reference) and is sent in periods within at least 0.7 seconds. The SCR time stamp is incorporated into the program stream packet and is only present in packets for transmitting the SCR. The time stamp in the transport packet stream is a PCR and is sent in periods within at least 0.1 seconds. The PCR time stamp is incorporated into the transport stream packet to send the PCR.
The PCR is a total of 42 bits comprised of a 9-bit program clock reference extension, and a 33-bit program clock reference base. The program clock reference extension counts from 0 to 299, and the program clock reference base is incremented by one bit on the program clock extension carrier. The MPEG-2 system clock operates at 27 MHz so that a 24-hour portion of time is counted as a 27 MHz clock unit on this 42-bit counter. The PCR in other words, is a PCR counter value (PCR value) counted by the system clock.
Three documents relating to this invention are listed below.
(1) M. Perkins and P. Skelly, xe2x80x9cA Hardware MPEG Clock Recovery Experiment in the Presence of ATM Jitterxe2x80x9d, ATM Forum contribution to the SAA sub-workinggroup, 94-0434, May, 1994.
(2) G. Franceschini, xe2x80x9cExtension of the Adaptive Clock Method to Variable Bit Rate Streamsxe2x80x9d, ATM Forum contribution to the SAA sub-working group, 94-0231, May, 1994.
(3) ISO/1EC13818-1(MPEG-2 Systemsxe2x80x9d, xe2x80x9cGENERIC CODING OF MOVING PICTURES AND ASSOCIATED AUDIOxe2x80x9d, Recommendation H.222.0, ISO/IEC JTC/SC29/G11NO721 rev, June 1984.
The above document (1) discloses hardware for achieving synchronization with the system clock of the information generator source, based on data simulating the jitter occurring on the ATM. The above document (2) discloses methods for synchronization relating to variable bit rates but does not discuss how to reduce delay distortion. The above document (3) is a draft of international standards relating to MPEG-2 system.
However, when the time stamp added by the encoder sent from the information generator source arrives at precisely the same period at the decoder, the decoder can easily synchronize with the encoder system clock by using the above disclosed methods. However, these methods assume as a necessary precondition that there is a fixed delay along the transmission path. The contents of the above document (3) in fact mention a fixed delay on the transmit path. Therefore, the following problems occur when a random delay such as on a network or namely, delay distortion is added, and there is also no means for rewriting the time stamp value to cope with a corresponding amount of delay distortion.
The time stamp value input to the PLL (Phase Locked Loop) frequency demodulator (hereafter called PLL) on the decoder side, has been added with the delay distortion, and the information source generator and decoder clock frequency differential. The PLL low-pass filter is designed to absorb the delay distortion however when the delay distortion cannot be adequately absorbed when large. Further problems are that long time is required for synchronizing when alleviating delay distortion in a PLL and the PLL circuit itself requires a complex design. A method not dependent on the decoder PLL is therefore required in order to absorb the delay distortion.
One example of this problem is when using an MPEG-2 transport stream on an ATM network. Since transmit from a plurality of nodes to a unidirectional node is performed by ATM switching per the statistical multiplexing characteristic of ATM, the cell is made to wait in the buffer. A delay is consequently generated and a random delay distortion is added to the ATM cell.
In this case, the ATM network performs switching based on the ATM header without being aware of the contents of the payload so that the timestamp is not rewritten. Therefore, the problem occurs that even if the ATM cell is assembled in the MPEG-2 system layer packet, the previously existing distortion delay is still propagated.
To resolve this problem, a buffer was installed on the receive side to absorb the distortion delay to some extent, per the VOD standards of the ATM forum, and the clock pulse was reproduced by a decoder PLL. A method referred to as the adaptive clock method is recommended.
Though this method was simple, since there was no control of synchronized data, a drawback was that jitter could not be completely eliminated in the network 14. The PLL response can be delayed in order to decode MPEG transport streams containing this jitter, however merely performing analog shaping of the jitter constituents will still allow jitter constituents to remain when viewed over a long time period.
A jitter for instance of 1 ms to 2 ms can only be reduced by approximately 40 xcexcs. Therefore, decoding an MPEG transport stream containing jitter, recording it on a VTR and viewing the image on a monitor presents no significant problem. However, when attempting to retransmit an MPEG transport stream sent over an ATM network, to a local station or a distant station, the jitter value cannot be limited to within MPEG standards and this jitter causes great damage on the ATM network. This method in other words, is not capable of achieving a jitter value within xc2x1500 ns, to constitute an allowable PCR value representing quality on a station quality level.
Also, when the encoder 11 encodes at a variable rate rather than a fixed rate, if the decoder does not know that the transmit rate has changed, then circuits to reduce delay distortion cannot function correctly so that the applicants proposed communication devices in Japanese Patent Application No. Hei 6-276324 and Japanese Patent Application No. Hei 7-78572 to store data indicating the packet arrival period in the packet, and based on this data, to correctly operate circuits to reduce the delay distortion.
Data indicating the packet arrival period, or for eliminating such jitter is a data amount from 1 to 8 bytes. However, the amount of data capable of being stored in the header of the transport stream packet is only up to one byte. Data in excess of one byte must be stored in a specially designated transport stream packet whose essential packet format has been altered.
This invention, in view of the circumstances of the related art, has the object of sending or receiving information for control exceeding one byte.
A communications device according to one aspect of the present invention, for transmitting data to a receiver by way of a network is characterized in having a conversion means for converting a first format data to a second format data comprised of control information inserted into the header or the trailer for controlling the operation of the receiver and, a transmission means to transmit the second format data to the receiver.
The transmission means according to the aspect of the present invention can split up the second format data into a plurality of packets and transmits the plurality of packets.
The control information can consist of synchronizing information utilized to control synchronization in the receiver.
The first format data is time based data containing time information for each specified timing, and the conversion means generates the synchronizing information based on the time information, and can insert the synchronizing information into the second format data as the control information.
The control information can be utilized as information for conveying the transmit rate.
According to another aspect of the present invention, a communications method is characterized by comprising a conversion step to convert a first format data to a second format data comprised of control information inserted into the header or the trailer for controlling the operation of the receiver and, a transmission step to transmit the second format data to the receiver.
According to another aspect of the present invention, a program of a record medium is characterized by comprising a conversion step to convert a first format data to a second format data comprised of control information inserted into the header or the trailer for controlling the operation of the receiver and, a transmission step to transmit the second format data to the receiver.
According to another aspect of the present invention, a communications device is characterized by comprising a receive means to receive from the network a first format data containing a header or a trailer, a conversion means to convert the first format data to second format data, a detection means to detect control information contained in the header or the trailer and, a control means to control the conversion means based on the control information.
The receive means is capable of receiving the first format data split up into a plurality of packets.
The control information can constitute synchronizing information utilized for control of synchronization.
The first format data is time-based data containing time information for each specific timing, the control means controls the conversion means based on the synchronizing information and, the conversion means converts the first format time information.
The control information is capable of converting information utilized for conveying the transmit rate.
A communications method according to another aspect of the present invention is characterized by comprising a receive step to receive first format data containing a header or a trailer from the network, a conversion step to convert the first format data to second format data, a detection means to detect control information contained in the header or the trailer and, a control step to control the conversion step processing based on the control information.
A program for a recording medium according to another aspect of the present invention is characterized by comprising a conversion step to convert a first format data to a second format data, a detection step to detect the control information contained in said header or said trailer of said first format data, and a control step to control the processing in said conversion step based on said control information.
A communications system according to another aspect of the present invention is characterized in that said transmitter is comprised of a first conversion means to convert a first format data containing a header or trailer into a second format data input in said header or trailer with control information to control operation of said receiver and, a transmission means to transmit said second format data to said receiver, and said receiver is comprised of a receive means to receive second format data containing said trailer or said header from said network, a second conversion means to convert said second format data to said first format data, a detection means to detect control information contained in said trailer or said header and, a control means to control the operation of said second conversion means based on said control information.
A communications device according to another aspect of the present invention and a communication method according to another aspect of the present invention, are characterized in that a first format data containing a header or trailer is converted into a second formation data having a trailer or a header inserted with control information to control the receiver operation, and the second format data is sent to the receiver.
A recording medium, according to another aspect of the present invention, is characterized in that a first format data containing a header or trailer is converted into a second format data inserted in the header or trailer with control information to control the receiver operation.
A communications device according to another aspect of the present invention and a communication method according to another aspect of the present invention, are characterized in that a first format data containing a header or trailer is received from a network, the first format data is converted into a second format data, control information contained in the header or trailer is detected, and conversion is controlled based on the control information.
A recording medium according to another aspect of the present invention is characterized in that the first format data is converted into a second format data, control information contained in the header or trailer is detected, and conversion is controlled based on the control information.
A communications system according to another aspect of the present invention is characterized in that the first format data containing a header or trailer, is converted into a second format data inserted in the header or trailer with control information to control the receiver operation, the second format data is transmitted to a receiver, the second format data containing the header or trailer is received from the network, the second format data is converted to a first format data, control information contained in the header or the trailer is detected, and the second conversion means is operated based on the control information.